Any kind of permanent, semi-permanent or temporary structure designed to attract fishes (specially tunas) and used in ocean purse seine fisheries. FADs can be moored or drifting devices and consist of surface or sub-surface buoys or floats to which underwater ropes, lines or chains can be attached. The most sophisticated type can be equipped with an echo sounder to estimate the biomass under it, a GPS to locate the device, and either a VHF, Immarsat or Iridium connection to periodically send the information to the vessel which owns it.